126 



blastoderm of the Chick" in the Proceedings of the Royal Society, 

 Vol. 60, 1896. 



In these I maintained that there are in the vertebrate embryo 

 "two main centres of growth, each in itself tending to produce a ra- 

 dially symmetrical form; but since these two centres of growth are 

 situated eccentrically to each other the resulting embryo is cylindrical, 

 and, subsequently bilaterally symmetrical" (Assheton, 2). And again, 

 "there is evidence to shew that the embryo (Frog) is derived from 

 two definite centres of growth, the first, and phylogenetically the older, 

 being a protoplasmic activity which gives rise to the anterior end of 

 the embryo (= gastrula stage); the second, which gives rise to the 

 growth in length of the embryo". So also in the same paper I wrote 

 concerning Amphioxus, p. 226: "The splitting process in the Frog 

 corresponds in results to the invagination process of Amphioxus, 

 while the overgrowth of certain parts of the white pole of the ovum 

 of the Frog by the dorsal, and subsequently lateral and ventral lips 

 of the blastopore, together with the continuation of this process in the 

 formation of the tail, corresponds to the elongation of the gastrula in 

 Amphioxus, by means of what Hatschek called the polar cells." 



Again in 1896, p. 353: "it seems clear that all those parts in 

 front of the first pair of mesoblastic somites (that is to say, the heart, 

 the brain, and medulla oblongata, the olfactory, optic and auditory 

 organs and fore gut) are developed from that portion of the unincub- 

 ated blastoderm which lies anterior to the centre of the blastoderm 

 and that all the rest of the embryo is formed by the activity of the 

 primitive streak area." 



These facts are illustrated by diagrams for the rabbit, Asshe- 

 ton (1), Plate 22; for the frog, Assheton (2), Plate 24, and for the 

 chick, KopscH (17), Fig. 1. 



The experiments themselves as regards the chick have been re- 

 peated and confirmed and extended by Peebles, Sumner, Kopsch, etc. 

 For the frog also Eycleshymee (8) supports this view. 



The recognition of the occurrence of these two centres of growth 

 and of the part which each separately plays in the production of cell 

 material for the growth of the embryo, is to my mind essential to the 

 correct appreciation of vertebrate embryology. That they were not 

 long ago recognised and appreciated is due, I think, very largely to 

 the disturbing effects of the theory, of formation of the vertebrats em- 

 bryo by concrescence of originally separats parts of the germ ring. 



These two centres of growth are now partially recognised by Hub- 

 RECHT — as they were also by Lwoff in 1894 although they were dif- 



